For volumetric flow measurement, magneto-inductive flow measuring devices utilize the principle of electrodynamic induction: charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in electrodes arranged essentially perpendicular to the flow direction of the medium and perpendicular to the direction of the magnetic field. The measurement voltage induced in the electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, and, thus, proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline or the measuring tube can be determined. The measurement voltage is usually tapped via the electrode pair, which is arranged in the region of the measuring tube in which the maximum magnetic field strength and, thus, the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; however, magneto-inductive flow measuring devices with contactless, capacitively coupling electrodes are also known.
The measuring tube can, in such case, be manufactured either from an electrically conductive material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, the portion coming in contact with the medium must then be lined with a liner made from an electrically insulating material. The liner is, depending on temperature and medium, for example, made of a thermoplastic, a thermosetting plastic or an elastomeric, synthetic material. However, magneto-inductive flow measuring devices with a ceramic lining are also known.
An electrode can essentially be subdivided into an electrode head, which at least partially comes in contact with a medium flowing through the measuring tube, and an electrode shank, which is almost completely encased in the wall of the measuring tube.
The electrodes are, along with the magnet system, the central components of a magneto-inductive flow measuring device. With regard to the embodiment and arrangement of the electrodes, it is to be ensured that they can be installed in the measuring tube as easily as possible, and that, subsequently during measurement operation, no leakage problems occur; moreover, the electrodes should be characterized by a simultaneously sensitive, and low-disturbance, registering of measurement signals.
In addition to the measuring electrodes, which serve for picking up a measurement signal, additional electrodes in the form of reference or grounding electrodes, which serve to measure an electrical reference potential or to detect partially filled measuring tubes, are often installed in the measuring tube.
U.S. Pat. No. 4,279,166 shows an electrode arrangement of a magneto-inductive flow measuring device for sealing an insert into the flow measuring device. The sealing is achieved, in such case, essentially by a threaded cap, which is screwed onto a screw thread of the insert, and the electrode is affixed in the insert. The arrangement is sealed by an O-ring placed under the shoulder of the electrode.
U.S. Pat. No. 5,224,394 also shows an arrangement for sealing electrodes inserted into the wall of a ceramic measuring tube of a magneto-inductive flow measuring device. For such purpose, the electrode is held in a particular position by a spring, so that the electrode seals off the measuring tube.
U.S. Pat. No. 3,171,990 shows an arrangement for securement of electrodes in a measuring tube. In such case, the electrode is inserted in an insert into the measuring tube, and affixed in assembly by means of a nut and a screw thread on the electrode shank.
These embodiments have, however, the disadvantage, that the radial bores in the measuring tube form weak points, which, in the case of measured substances flowing through under high pressure, lead to cracks and failures. Furthermore, in the case of small nominal diameters of measuring tubes, it is difficult to install the electrodes in the measuring tube and to seal the bores.
Additionally, in the case of measuring tubes made of plastic or synthetic material, the conventional types of securement of electrodes in the measuring tube used in the state of the art are not practical.